1. Field of the Invention
This invention relates to a method for the production of clad glass rod of substantially uniform diameter and to clad glass rod produced by such a method.
2. Description of the Prior Art
It is known that clad glass rod i.e., glass rod in which a core rod of glass has a coating or cladding of a second glass of different composition and optical properties to that of the core glass, can be redrawn into a clad fibre or use as an optical light guide in fibre optics.
In operating a process in which optical fibre is drawn from pre-formed clad glass rod, it is desirable to ensure that no contamination has occurred during manufacture of the clad rod, and that the rod is of a substantially uniform diameter.
Our U.S. Pat. No. 3,726,656, relates to a method of production of clad glass rod in which contamination at the interface between the core glass and the cladding glass is minimal, and required optical properties are achieved. An elongated clad glass member is produced by supporting within the same container a molten layer of a first glass on a molten layer of a second glass, the two layers being provided by two quiescent pools of glass, one overlying the other within the same container, drawing from the free surface of the upper layer an elongated member comprising an outer cladding of the first glass on a core of the second glass which is entrained at the interface between the layers, and stabilising the elongated member by cooling above the surface of the upper layer. A cooler, e.g. a water jacket, is preferably disposed as near as possible to the free surface of the glass to effect such cooling, so that the elongated member, i.e., the clad glass rod which is drawn upwardly is quickly set.
As well as the relative viscosities of the two glasses and the speed of upward drawing, other factors which control the overall diameter of the clad rod and the radial width of the cladding are the distance of the bottom of the cooler from the free surface of the upper layer of glass, and the intensity of cooling.
When operating a batch process for the production of clad glass rod, the level of molten glass in the reservoir or pot from which the rod is being drawn falls as the rod is formed, and it is consequently necessary to drive the cooling means down toward the molten glass as the glass level in the reservoir or pot falls.
It is an object of the present invention to provide a method of forming clad glass rod in which an improved control of diameter of the rod is effected during the drawing of the rod.